1. Field of the Invention
The present invention relates to an optical communication apparatus, and particularly to an optical modulation method using a Mach-Zehnder optical modulator, which is designed for a time division multiplexing optical transmitter in an optical transmission system.
2. Description of the Related Art
As the amount of information has significantly increased, demand for a large-capacity and long-distance optical communication system has increased in recent years. An optical amplification relay system having a transmission rate of 10 Gbit/s is now being put to practical use. The need for an optical communication system having a larger capacity is expected to increase hereafter, and both a time division multiplexing (TDM) optical communication system and a wavelength division multiplexing (WDM) optical communication system are in the research and development stage.
To achieve a large-capacity optical communication system such as that having a transmission rate of 40 Gbit/s by means of the time division multiplexing (TDM) system, some technical problems must be solved, including the following two major problems:
(1) Implementing electronic and optical devices operating at very high speed in an optical transmitter/receiver.
(2) Overcoming factors restricting transmission distance of a transmission line fiber (chromatic dispersion, nonlinear effect, and polarized wave dispersion).
As to (1), the operating speed of an optical communication system is restrained by the limit of the operating speed of an electronic device rather than an optical device, at present. To overcome this problem, some methods were devised, including an optical time division multiplexing (OTDM) modulation method (G. Ishikawa et al., ECOC ""96 Post-deadline papers, TuC 3.3, pp. 37-40, 1996), which generates an optical signal of 40 Gbit/s by using only an electronic device having a band of 20 Gbit/s and by multiplexing light on a time axis.
Regarding chromatic dispersion (i.e. GVD) as set forth in (2), dispersion tolerance is in inverse proportion to the square of a bit rate. A 10 Gbit/s system has a dispersion tolerance of approximately 800 ps/nm, while the dispersion tolerance of a 40 Gbit/s system is one-sixteenth of that of the 10 Gbit/s system, that is, approximately 50 ps/nm. Narrowing the band of a signal light spectrum is effective in reducing deterioration in a waveform caused by chromatic dispersion. As one of the methods to attain this, an optical duo-binary modulation method is in the research and development stage. (For example, A. J. Price et al., xe2x80x9cReduced bandwidth optical digital intensity modulation with improved chromatic dispersion tolerancexe2x80x9d, Electron. Lett., Vol. 31, No. 1, pp. 58-59, 1995)
In this method, narrow-band ternary electric driving signals are generated by causing an electric input signal to pass through a low-pass filter for a band which is approximately one-fourth of a bit rate. Furthermore, an optical modulation is performed with amplitude, which is double the amplitude for the voltage (Vxcfx80 voltage) at which light is turned on/off in the Mach-Zehnder modulator, and driving is performed so that electric signals of 3 values, xe2x80x9c0xe2x80x9d, xe2x80x9c0.5xe2x80x9d, and xe2x80x9c1xe2x80x9d correspond to optical signals of values, xe2x80x9cxe2x88x921xe2x80x9d, xe2x80x9c0xe2x80x9d, and xe2x80x9c1xe2x80x9d, respectively. Therefore, in optical modulation, the bandwidth of a signal spectrum is reduced to approximately half, and dispersion tolerance can be increased to double or more of that in the NRZ modulation method. When a large capacity is achieved by the wavelength division multiplexing (WDM) technique, signal channels can be arranged more densely within the bandwidth of amplification by an optical amplifier by using a narrow-band spectrum of an optical signal such as that used in the optical duo-binary modulation method. However, the optical duo-binary modulation method has some problems, as follows. The optical duo-binary modulation method requires a high-speed electric device having the same bit rate as that of an optical signal, as the NRZ modulation method requires. To ideally perform push-pull driving at both the electrodes, the amplitude and phase of two driving signals must coincide precisely, resulting in that an electric driving system must meet strict demands. Furthermore, intersymbol interference of a waveform is large since, in principle, an optical modulation waveform is asymmetrical with respect to the upper half and lower half, and the cross point of signal waveforms in an eye-opening diagram is on the ON-side above the central level.
FIGS. 1 through 4E show one of the modulation methods to solve the above described problems. For further details regarding such a modulation method, refer to Japanese Patent Laid-open No. 3-200923 in publication.
First, as shown in FIG. 1, a binary electric signal having a bit rate of B/2(b/s) is inputted, as an input signal #1, to one of the input electrodes of a double-sided electrode Mach-Zehnder optical modulator. The input signal #1 is amplified by an amplifier AMP 1, and is applied to a double-sided drive Mach-Zehnder optical modulator, as an electric signal E1t. A binary electric signal (input signal #2) having a bit rate of B/2(b/s) is amplified by an amplifier AMP 2, delayed by a half-bit shift delay element (T/2 delay element) by half a bit, and applied to the other input electrode of the double-sided drive Mach-Zehnder optical modulator. Continuous (CW) light is inputted, as E0, to an optical input terminal of a push-pull type Mach-Zehnder optical modulator. This CW light is modulated by input signals E1(t) and E2(t) which are inputted to the push-pull type Mach-Zehnder optical modulator, and a modulated optical signal P(t) is outputted. The bit rate of the modulated optical signal P(t) is B(b/s).
FIG. 2 shows the operation of the double-sided drive Mach-Zehnder optical modulator using a half-bit shift multiplexing method.
As shown in a voltage-to-light intensity graph in FIG. 2, the light intensity in the double-sided drive Mach-Zehnder optical modulator periodically varies in accordance with a trigonometric function, depending on the difference between voltages applied to both the electrodes. When light intensity modulation is performed, an electric signal within a voltage range of Vxcfx80, at which light intensity becomes xe2x80x9c0xe2x80x9d, andxe2x80x94Vxcfx80, including a voltage (xe2x80x9c0xe2x80x9d voltage in FIG. 2) at which light intensity becomes xe2x80x9c1xe2x80x9d as the center of the range, is used. Intensity-modulated light, the light intensity of which varies between an extinction state (a state of xe2x80x9c0xe2x80x9d) and a state of maximum intensity (a state of xe2x80x9c1xe2x80x9d), is obtained by causing the voltage which is applied to both the electrodes to vary between Vxcfx80 and xe2x88x92Vxcfx80.
Alternatively, a voltage signal within a voltage range of Vxcfx80, at which light intensity becomes xe2x80x9c1xe2x80x9d, andxe2x80x94Vxcfx80, including a voltage at which light intensity becomes xe2x80x9c0xe2x80x9d as the center of the range, may be used as used in a modulation method 2 as shown in FIG. 3. A modulation method 1 is described below.
FIGS. 4A through 4E show the half-bit shift modulation method.
FIG. 4A shows the light intensity of CW light which is continuous light having a constant amplitude (light intensity). FIG. 4B illustrates a variation in amplitude of the input electric signal E1(t) as shown in FIG. 1. FIG. 4B assumes that an NRZ signal (1100100) is inputted, the bit rate of the NRZ signal is B/2(b/s), and the time slot length of one symbol is 2T=2/B(sec). Meanwhile, FIG. 4C illustrates a variation in amplitude of the input electric signal E2(t) as shown in FIG. 1. FIG. 4C assumes that an NRZ signal (01001010) is inputted, the bit rate of the NRZ signal is the same as that of the input electric signal #1, and the time slot length of one symbol is 2T=2/B(sec). Light modulation is affected by the difference between the above described input electric signals which are applied to both the electrodes of the double-sided drive Mach-Zehnder optical modulator. FIG. 4D shows the difference between the amplitudes of the input electric signals E1(t) and E2(t). As shown in FIG. 4D, an electric signal having a bit rate of B(b/s) (the time slot length of one symbol is given by: T=1/B(sec)), which is twice as large as the bit rate of the input electric signals E1(t) and E2(t), is applied to the push-pull type Mach-Zehnder optical modulator. FIG. 4E shows how CW light (E0 as shown in FIG. 4A) is modulated when the voltage difference as shown in FIG. 4D is applied to both the electrodes of the push-pull type Mach-Zehnder optical modulator. When the voltage difference as shown in FIG. 4D is Vxcfx80 or xe2x88x92Vxcfx80, the intensity of an optical signal outputted from the push-pull type Mach-Zehnder optical modulator is xe2x80x9c0xe2x80x9d, as shown in FIG. 2. Therefore, as shown in FIG. 4E, the intensity of an optical signal P(t) is in the extinction state (light intensity is xe2x80x9c0xe2x80x9d) when the voltage difference, as shown in FIG. 4D, is Vxcfx80 or xe2x88x92Vxcfx80. However, as is clear from FIG. 2, an optical signal P(t) having the maximum light intensity is outputted when the voltage difference is xe2x80x9c0xe2x80x9d. Thus, an optical signal P(t) having a desired bit rate (a bit rate becomes B(b/s)) can be generated by combining electric signals having a bit rate of B/2(b/s). For example, an electronic device having an operating rate of 20 Gbit/s is sufficient to generate a 40 Gbit/s optical signal, thereby significantly reducing demand for an electric driving system. This modulation method is referred to as the xe2x80x9chalf-bit shift multiplexing modulation methodxe2x80x9d hereinafter.
FIG. 5 shows a configuration of the double-sided electrode Mach-Zehnder optical modulator for use in the half-bit shift multiplexing modulation method of prior art, which is the same as that for use in NRZ modulation.
Two independent electric signals #1 and #2 having amplitude of Vxcfx80 and a bit rate of B/2(b/s) with a delay of a half time slot between each, are inputted to two signal electrodes #1 and #2 of the double-sided electrode Mach-Zehnder optical modulator. The Mach-Zehnder optical modulator has an applied voltage-to-light intensity characteristic that an applied voltage and light intensity periodically vary in accordance with a trigonometric function, as shown in FIG. 2. Here, the applied voltage corresponds to the potential difference between the input electric signals #1 and #2.
The half-bit shift multiplexing modulation method has the advantage that modulation can be performed only by using a slow-speed electronic device, resulting in that dispersion tolerance becomes large because a signal spectrum band becomes narrow. However, there is a problem that when half-bit shift multiplexing modulation is performed by using the conventional Mach-Zehnder optical modulator of a double-sided drive and electrode configuration as shown in FIG. 5, significant waveform deterioration arises compared to other modulation methods (optical duo-binary and NRZ) if chromatic dispersion exists in optical paths. This problem arises because in the half-bit shift multiplexing modulation method, voltages which are not in synchronization with each other are applied to both electrodes of the Mach-Zehnder optical modulator, resulting in two separate signals, thereby causing substantially the same operation as that in the case of combining two one-sided electrode drive Mach-Zehnder optical modulators.
FIG. 6 shows the relationship between a total chromatic dispersion amount and an eye-opening penalty in each modulation method.
FIG. 6 shows transmission waveform simulation in which deterioration in an optical signal that is received after being transmitted through an optical fiber is examined in an eye pattern by changing the length of the optical fiber having prescribed dispersion characteristics. A back-to-back eye-opening degree in the NRZ modulation method is used as a common criterion of a chromatic dispersion-to-eye-opening penalty in the transmission waveform simulation. Here, the term xe2x80x9cback-to-backxe2x80x9d refers to a state in which there is no optical fiber between a transmitter and a receiver, and substantially refers to an input signal itself. It is understood from FIG. 6 that dispersion tolerance (for example, a range of transmission line dispersion, in which the eye-opening penalty is 1 dB or less) in the half-bit shift multiplexing modulation method is small compared with other modulation methods (optical duo-binary and NRZ).
FIG. 7 shows an equalized waveform indicating deterioration in a waveform at the receiving end in each modulation method.
In the half-bit shift multiplexing modulation method, deterioration in a waveform for both positive and negative transmission line dispersion is significant and dispersion tolerance (for example, a range of transmission line dispersion, in which the eye-opening penalty is 1 dB or less) becomes small, compared with the NRZ modulation method and the optical duo-binary modulation method. Note that FIG. 7 shows the waveform in the half-bit shift multiplexing modulation method and NRZ modulation method for the band of 0.67 B and the waveform in the optical duo-binary modulation method for the band of 0.25 B. This is because transmission waveform simulation was performed for a band which causes the least deterioration in a waveform since it is well-known that 0.25 B is an optimum band for the optical duo-binary modulation method.
The reason why the half-bit shift multiplexing modulation method causes greater waveform deterioration, compared with other modulation methods, is explained with formulas below. When the electric field component of coherent (CW) light inputted to an optical modulator is given as
Ein=E0xc2x7exp(ixcfx890t)
(i is the imaginary unit, and xcfx890 is the angular frequency of a carrier),
the electric field components of the optical signal undergoing phase modulation xcfx86A(t) and xcfx86B(t) by a driving electric signal at signal electrodes #1 and #2 of the optical modulator are represented as follows:
signal electrode #1: E0/2xc2x7exp(i(xcfx890t+xcfx86A))
signal electrode #2: E0/2xc2x7exp(i(xcfx890t+xcfx86B))
The electric field component of an optical signal outputted from the optical modulator can be represented as follows:                               E          out                =                                                                              E                  0                                /                2                            ·                              exp                ⁡                                  (                                      i                    ⁡                                          (                                                                                                    ω                            0                                                    ⁢                          t                                                +                                                  φ                          A                                                                    )                                                        )                                                      +                                                            E                  0                                /                2                            ·                              exp                ⁡                                  (                                      i                    ⁡                                          (                                                                                                    ω                            0                                                    ⁢                          t                                                +                                                  φ                          B                                                                    )                                                        )                                                              =                                                                      E                  0                                /                2                            ⁢                                                {                                                            exp                      ⁡                                              (                                                  i                          ⁢                                                      xe2x80x83                                                    ⁢                                                      φ                            A                                                                          )                                                              +                                          exp                      ⁡                                              (                                                  i                          ⁢                                                      xe2x80x83                                                    ⁢                                                      φ                            B                                                                          )                                                                              }                                ·                                  exp                  ⁡                                      (                                          i                      ⁢                                              xe2x80x83                                            ⁢                                              ω                        0                                            ⁢                      t                                        )                                                                        =                                                            E                  0                                /                2                            ⁢                                                {                                                            (                                                                        cos                          ⁢                                                      xe2x80x83                                                    ⁢                                                      φ                            A                                                                          +                                                  cos                          ⁢                                                      xe2x80x83                                                    ⁢                                                      φ                            B                                                                                              )                                        +                                          i                      ·                                              (                                                                              sin                            ⁢                                                          xe2x80x83                                                        ⁢                                                          φ                              A                                                                                +                                                      sin                            ⁢                                                          xe2x80x83                                                        ⁢                                                          φ                              B                                                                                                      )                                                                              }                                ·                                  exp                  ⁡                                      (                                          i                      ⁢                                              xe2x80x83                                            ⁢                                              ω                        0                                            ⁢                      t                                        )                                                                                                          (        1        )            
Therefore, the intensity P(t) and phase xcfx86(t) of the output optical signal are represented as follows:
P(t)=(E0/2)2xc2x7{(cos xcfx86A+cos xcfx86B)2+(sin xcfx86A+sin xcfx86B)2}=E0/2{1+cos(xcfx86Axe2x88x92xcfx86B)}=E02xc2x7cos2((xcfx86Axe2x88x92xcfx86B)/2}xe2x80x83xe2x80x83(2)
"PHgr"(t)=tanxe2x88x92{(sin xcfx86A+sin xcfx86B)/(cos xcfx86A+cos xcfx86B)}=tanxe2x88x92{sin((xcfx86A+xcfx86B)/2)/cos((xcfx86A+xcfx86B)/2)}=(xcfx86A+xcfx86B)/2xe2x80x83xe2x80x83(3)
Wavelength chirping xcex94xcex is represented as follows:
xcex94xcex(t)=xcex94(2xcfx80cxe2x80x2/xcfx89)=xe2x88x92(2xcfx80cxe2x80x2/xcfx892)xc2x7xcex94xcfx89=xe2x88x92(2xcfx80cxe2x80x2/xcfx892)xc2x7dxcfx86(t)/dt=xe2x88x92(xcfx80cxe2x80x2/xcfx892)xc2x7(dxcfx86A/dt+dxcfx86B/dt)xe2x80x83xe2x80x83(4)
(cxe2x80x2: propagation velocity of light within an optical fiber)
The optical modulator, having a double-sided drive configuration, for use in the conventional NRZ modulation method as shown in FIG. 5 keeps the above described chirping constant, at 0, by using push-pull drive (by using two signals, which are the inverse of one another, as electric signals to be inputted to both sides). That is,
xcfx86A(t)=xcfx80/2xc2x7E(t)
xcfx86B(t)=xe2x88x92xcfx80/2xc2x7E(t)
E(t): input electric signal ON: E=1, OFF: E=0
P(t)=E02/2xc2x7{1+cos(xcfx80xc2x7E(t))}xcex94xcex(t)=0
When the half-bit shift multiplexing modulation is performed by using the conventional double-sided drive optical modulator having the same configuration, independent electric signals E1(t) and E2(t) are inputted to both the electrodes. Therefore, unlike the above described case in which the push-pull drive is used, phase modulation is represented as follows:
xcfx86A(t)=xcfx80xc2x7E1(t)xe2x80x83xe2x80x83(5)
xcfx86B(t)=xcfx80xc2x7E2(t)xe2x80x83xe2x80x83(6)
P(t)=E02/2xc2x7{1+cos(xcfx80xc2x7(E1(t)xe2x88x92E2(t)))}xe2x80x83xe2x80x83(7)
xcex94xcex(t)=xe2x88x92(xcfx802cxe2x80x2/xcfx892)xc2x7(dE1(t)/dt+dE2(t)/dt)xe2x80x83xe2x80x83(8)
E1(t): input electric signal #1 ON: E=1, OFF: E=0
E2(t): input electric signal #2 ON: E=1, OFF: E=0
From expression (7), optical signal intensity P(t) depends only on E1(t)xe2x88x92E2(t), which is the difference between the input electric signals E1(t) and E2(t). Table 1 shows the optical signal intensity P(t) as the combination of E1(t) and E2(t).
In contrast, it is understood from expression (8) that the wavelength chirping xcex94xcex(t) depends on dE1(t)/dt, which is a change in intensity of the input electric signal E1(t) at the time of rise and fall, and dE2(t)/dt, which is a change in intensity of the input electric signal E2(t) at the time of rise and fall, resulting in two types of chirping at the time of a rise (xe2x80x9c0xe2x80x9dxe2x86x92xe2x80x9c1xe2x80x9d) of the optical signal P(t) generated, specifically, chirping to a long-wavelength side (xcex94xcex greater than 0) and chirping to a short-wavelength side (xcex94xcex less than 0). In other words, and as shown in the table above, the following four combinations exist in the case of the rise of an optical signal.
(1) While E1(t) remains unchanged at 0, E2(t) changes from 1 to 0: xcex94xcex greater than 0
(2) While E1(t) remains unchanged at 1, E2(t) changes from 0 to 1: xcex94xcex less than 0
(3) While E2(t) remains unchanged at 0, E1(t) changes from 1 to 0: xcex94xcex greater than 0
(4) While E2(t) remains unchanged at 1, E1(t) changes from 0 to 1: xcex94xcex less than 0
Similarly, two types of chirping, specifically, chirping to the long-wavelength side (xcex94xcex greater than 0) and chirping to the short-wavelength side (xcex94xcex less than 0) occurs at the time of a fall (xe2x80x9c1xe2x80x9dxe2x86x92xe2x80x9c0xe2x80x9d) of the optical signal P(t). FIG. 8 gives an example of the same signal patterns as those shown in FIGS. 4A through 4E, and shows the change in the optical signal intensity P(t) and chirping xcex94xcex(t) with the passage of time. It is understood from FIG. 8 that the chirping to a long-wavelength side (xcex94xcex greater than 0) and chirping to a short-wavelength side (xcex94xcex less than 0) occurs together at the time of both the rise and fall of an optical signal. One of the reasons why dispersion tolerance is very small in the half-bit shift multiplexing modulation method is that since the chirping of xcex94xcex greater than 0 and chirping of xcex94xcex less than 0 occurs together in the transmission of a signal, this intricately affects the dispersion characteristics of the optical fiber.
As described above, a fiber transmission line has a characteristic that is called chromatic dispersion.
Chromatic dispersion D (ps/nm) greater than 0xe2x86x92The longer the wavelength, the lower the group velocity of the optical signal.
Chromatic dispersion D (ps/nm) less than 0xe2x86x92The longer the wavelength, the higher the group velocity of the optical signal.
Therefore, in the case of coexistence of the chirping to a long-wavelength side (xcex94xcex greater than 0) and chirping to a short-wavelength side (xcex94xcex less than 0) at the time of the rise and fall of the eye of an optical signal, significant deterioration in a waveform occurs because a difference in group delay between the two types of chirping arises, resulting in that a waveform is divided in two.
As explained above, to break the present limitations of a high-speed optical communication, a high-speed optical signal must be generated by using an electric circuit having a low operating speed. Therefore, it is desirable to use the half-bit shift multiplexing modulation method, which enables generation of an optical signal having a bit rate being twice as large as that of an electric signal. However, in the half-bit shift multiplexing modulation method, complex wavelength chirping arises, and significant deterioration in a waveform occurs in the transmission of a signal through an optical fiber having dispersion characteristics. This causes some problems, such as the inability to properly receive a signal at the receiving end and the reception of a signal with many errors.
An object of the present invention is to provide a high-speed optical communication apparatus and method thereof which enable preventing deterioration in the waveform of an optical signal.
The optical modulation apparatus of the present invention comprises a first electrode unit which applies a first driving signal having a prescribed bit rate to a first light path of a Mach-Zehnder optical modulator; a second electrode unit which applies a second driving signal having the prescribed bit rate and a phase which differs from that of the first driving signal by a half time slot, to a second light path of the Mach-Zehnder optical modulator; and a third electrode unit which performs phase modulation for light being transmitted through the light paths provided in the Mach-Zehnder optical modulator so that chirping imparted by the first driving signal to light passing through the first light path and chirping imparted by the second driving signal to light passing through the second light path are offset by each other after the light passing through the first light path and the light passing through the second light path are coupled. The optical modulation apparatus of the present invention is characterized by increasing dispersion tolerance, because of the bit rate of modulated light, which is obtained by applying the first driving signal and second driving signal, being double the prescribed bit rate and because of a ternary optical signal.
The demodulation apparatus of the present invention comprises a photoelectric conversion unit which converts an optical signal received, into an electric signal; an edge detecting unit which detects the rise and fall of the electric signal; an even/odd-numbered edge detecting unit which detects an even-numbered edge signal that is in an even-numbered time position and an odd-numbered edge signal that is in an odd-numbered time position, assuming that all the time positions that are set at intervals of one time slot of the electric signal are numbered; and a demodulation unit which regenerates the first driving signal and the second driving signal which were applied to the optical modulator at the transmitting end, by inverting a first output signal and a second output signal by using the even-numbered edge signal and the odd-numbered edge signal, respectively.
The optical modulation method of the present invention, which is designed for the Mach-Zehnder optical modulator, comprises the steps of: (a) applying a first driving signal having a prescribed bit rate to a first light path of the Mach-Zehnder optical modulator; (b) applying a second driving signal having the prescribed bit rate and a phase which differs from that of the first driving signal by a half time slot, to a second light path of the Mach-Zehnder optical modulator; and (c) performing phase modulation for light being transmitted through the light paths provided in the Mach-Zehnder optical modulator so that chirping imparted by the first driving signal to light passing through the first light path and chirping imparted by the second driving signal to light passing through the second light path are offset by each other after the light passing through the first light path and the light passing through the second light path are coupled. The optical modulation method of the present invention is characterized by increasing dispersion tolerance, because of the bit rate of modulated light, which is obtained by applying the first driving signal and second driving signal, being double the prescribed bit rate and because of a ternary optical signal.
The demodulation method of the present invention comprises the steps of: (a) converting an optical signal received, into an electric signal; (b) detecting the rise and fall of the electric signal; (c) detecting an even-numbered edge signal that is in an even-numbered time position and an odd-numbered edge signal that is in an odd-numbered time position, assuming that all the time positions that are set at intervals of one time slot of the electric signal are numbered; and (d) regenerating the first driving signal and the second driving signal which were applied to the optical modulator at the transmitting end, by inverting a first output signal and a second output signal by using the even-numbered edge signal and the odd-numbered edge signal, respectively.
According to the present invention, by using two independent driving signals having data to be transmitted, an optical signal, the bit rate of which is double that of the driving signals, can be generated, and the use of the ternary optical signal enables increasing dispersion tolerance. Furthermore, the present invention solves the problem of small dispersion tolerance in the optical modulation, caused by significant deterioration in a waveform of a generated optical signal during transmission through an optical fiber under the influence of complex chirping, because the present invention increases dispersion tolerance and achieves long-distance transmission by providing an electrode unit which offsets chirping in a modulation process. Therefore, a high-speed optical signal can be generated by using driving signals generated in a low-speed electric circuit, and can be transmitted for a long distance.
At the receiving end, the optical signal transmitted, as described above, can be demodulated by using the characteristics of the optical modulation method, and the two independent driving signals having data can be regenerated relatively easily.